Wireless communication systems often use multiple antennas to improve the system performance. For example, multiple transmit antennas may be used for beam-forming, diversity, and/or Multiple Input Multiple Output schemes to implement spatial multiplexing. In each of these cases, the signals to be transmitted from each antenna are multiplied by an antenna weight, which is often a complex number. The radiation characteristics of the set of antennas used to transmit a certain signal, and the corresponding antenna weights, will determine how the signal is radiated into space, e.g., in what direction(s), with what power, etc. Many of these applications require feedback from the receiving node to the transmitting node. For example, MIMO schemes may require that the transmitting node has full knowledge of the MIMO radio channel or other information that can be used to control the antennas and/or antenna weight selection. To that end, the receiving node may provide channel state information, antenna weight information, pre-coder index information, etc., to the transmitting node.
While multiple antenna solutions have been effectively implemented with two, four, or even eight antennas, using more than eight antennas traditionally increases the overhead required to implement the desired multiple antenna solution. This problem is caused, for example, by the limits associated with the number of pilot signals that can be transmitted within the coherence time of a radio channel, which sets an upper bound to the meaningful number of antennas that can be used. Further, higher numbers of transmission antennas result in narrower radiation patterns, which are generally very sensitive to channel estimation error, quantization errors, and channel variability, e.g., such as caused by the combined effects of multipath propagation and mobility of the transmitting node, the receiving node, and/or other objects within the propagation environment. In addition, the current trend of increasing bandwidths results in more frequency selective channels. Thus, antenna weights used to generate the desired beams are more likely to vary over the frequency band of interest.
Conventional approaches to multi-antenna communications rely on signal quality information, e.g., the signal-to-noise ratio (SNR), and can be limited by the rate at which such information is fed back to the transmitting node. Thus, there remains a need for alternative solutions that reduce the amount of feedback and/or feedback rates.